1. Field of the Invention
The present invention relates to methods for preparing alkali cellulose and cellulose ether.
2. Description of the Related Art
For the preparation of cellulose ether, known is a method comprising steps of bringing an aqueous alkali solution into contact with highly purified pulp to prepare alkali cellulose and etherifying the resulting alkali cellulose with an etherifying agent. Although the cellulose ether obtained as a final product becomes soluble in water by properly controlling the degree of substitution, a water insoluble portion existing in the solution sometimes lowers the transmittance of the aqueous solution or damages the commodity value of the cellulose ether as a contaminant.
The insoluble portion appears owing to the existence of a low substituted portion, that is, a portion not having enough substituent to permit dissolution of the cellulose ether in water. One of the causes of the insoluble portion is uneven alkali distribution in the alkali cellulose.
Roles which the alkali cellulose plays can include swelling the cellulose to change its crystal structure in the pulp, thereby promoting the penetration of the etherifying agent; catalyzing the etherification reaction with alkylene oxide; or serving as a reactant for an alkyl halide.
A portion of pulp not brought into contact with the aqueous alkali solution is not involved in a reaction and therefore remains as an insoluble portion. The uniformity of the alkali in alkali cellulose directly affects the amount of the insoluble portion.
According to a preparation method of alkali cellulose, pulp is dipped in an aqueous solution of sodium hydroxide to allow the pulp to sufficiently absorb the solution, and the resulting pulp is pressed to remove the excess alkali so as to leave a predetermined alkali amount.
This method is, however, disadvantageous from the standpoint of productivity because it needs cumbersome operations and in addition, the alkali amount cannot easily be adjusted to a predetermined value.
A method of adding a predetermined amount of alkali to the pulp powder obtained by pulverization, and then mixing them mechanically is excellent in productivity because this method facilitates control of the alkali concentration to a predetermined value and can be composed of one step. The mechanical mixing can include a batch process in which pulp powder and an aqueous solution of sodium hydroxide are charged in a double-shaft kneader. In this kneader type mixer, however, an increase in the mixed volume leads to a decrease in the contact area of an agitating blade with the mixture, thereby the mixing ability being lowered. Moreover, it requires a lot of time to obtain a relatively uniform mixture and the scale of the equipment must inevitably be enlarged.
In a mixer equipped with a proshear agitating blade and a chopper, an increase in the equipment size makes it difficult for the chopper to apply a shear force to a mixture as in the above-described kneader so that a mixture uneven at the micro level is produced. In addition, the mixer has to be large in the size and imposes a heavy burden from the viewpoints of space and cost.
The mixer with a proshear agitating blade has two types, that is, a batch type and a continuous type. A back flow of the mixture from the outlet of alkali cellulose to the inlet of pulp cannot be prevented completely and the mixture containing an uneven portion is sometimes discharged. In such mechanical mixing, it is difficult to obtain a uniform mixture, in a true sense, of cotton-like powdered pulp and a relatively small amount of an alkali. The cellulose ether prepared from such alkali cellulose inevitably contains a larger amount of an undissolved portion compared with that prepared by the above-described dipping method.
For improving the uniformity of alkali, an increase in the volume of an aqueous alkali solution by using a dilute aqueous alkali solution was investigated. However, it is not preferred because in the subsequent etherification reaction, water in the reaction system causes an undesirable side reaction with an etherifying agent and markedly decreases the efficiency of primary reaction between the etherifying agent and the cellulose. Removal of water prior to the etherification reaction-is possible, but unpractical because a vapor pressure of the aqueous alkali solution is very low compared with that of water.
Another possible method is that of increasing the volume of a mixture of an aqueous alkali solution and alcohol by using a lower primary alcohol having compatibility with the aqueous alkali solution as described in Japanese Patent No. 132579. This method requires removal of the lower primary alcohol prior to the etherification reaction because of a side reaction between the alcohol with the etherifying agent.
A method for improving the uniformity of alkali cellulose even when the above-described mixer or a vertical mixer equipped with a simple agitating blade is employed has been investigated, the method comprising use of a large amount of an inactive dispersant such as dimethoxyethane or dimethyl ether (Japanese Patent Application Unexamined Publication Nos. 56-16501/1981 and 58-103501/1983). However, this method requires, after the preparation of alkali cellulose, removal of this dispersant prior to the etherification reaction or during purification after the reaction. The removal makes the method cumbersome and inevitably raises the production cost.
In Japanese Patent Application Unexamined Publication No. 2001-302701, proposed is a method of using alkali cellulose which has been prepared by continuously feeding pulp powder, which has been obtained by pulverizing pulp, to a double-shaft kneader and mixing the pulp powder with an aqueous alkali solution fed simultaneously and continuously from the same feed port or another position. Even in this method, however, the conditions under which the pulp powder and the aqueous alkali solution are mixed are not constant. The concentration of the aqueous alkali solution changes by adding the pulp powder thereto. Thus, such two reactants, which have encountered at an uneven concentration, are forcibly mixed in the double-shaft kneader so that the alkali concentration does not become completely uniform in the pulp powder.